Challenges in Containers

Sometimes you get challenges provided with a Dockerfile. In most cases, it's best to use it, as you can be sure it acts the same locally and remotely.

Unfortunately, that can be rough. There are a few steps. In essence, we want to use gdbserver to set up a debug session, then connect to gdbserver from our host to leverage the full power of whatever we want to debug with. These steps work for debugging a binary hosted via socat.

Quick Copy

Add:

RUN apt-get install -y gdb gdbserver
 -- OR --
RUN apk add gdb

-p 9090:9090 --cap-add=SYS_PTRACE

Run:

docker exec -it challenge /bin/bash
gdbserver :9090 --attach $(pidof challenge)

Connect:

r2 -d gdb://localhost:9090

OR

gdb challenge
target remote :9090

Explanation

Install

Add some installs to the Dockerfile:

RUN apt-get install -y gdb gdbserver

If the Dockerfile is an alpine image, instead use

RUN apk add gdb

gdbserver is automatically installed as part of the package.

Change Run Command in build_docker.sh

Add the

-p 9090:9090 --cap-add=SYS_PTRACE

flags to the docker run ... command in build_docker.sh.

• -p 9090:9090 binds the internal port 9090 to the external port 9090, so we can connect to localhost:9090 for the gdbserver

• --cap-add=SYS_PTRACE gives the container the capability to ptrace a process, which we need for debugging. The alternative is to run it in --privileged mode, which is far more unsafe

Start the Executable and get the PID

Get a shell with docker exec:

docker exec -it challenge /bin/bash

Note that to get a binary started with socat, we have to connect to the service first in order to start a process. So, outside the container, connect with nc:

$ nc localhost 1337
<pwnable binary>

Don't end the process. Switch back to the Docker root shell:

root@096c4ec3bca6:/# pidof challenge
22

Grab the PID of the subprocess, in this case 22.

Starting GDBserver

Now start a gdbserver:

gdbserver :9090 --attach 22

And on your host you can now connect to it with radare2 or GDB:

$ r2 -d gdb://localhost:9090

$ gdb challenge
(gdb) target remote :9090
Remote debugging using 172.17.0.2:9090
[...]
(gdb)

And boom.

Note the issue is that you have to restart gdbserver every time you connect again. Don't forget! Maybe there's a better way, but I don't know.

Did try and replace the shell commands with a single docker exec, but the $() is resolved before it is piped to the Docker:

$ docker exec -it challenge gdbserver :9090 --attach $(pidof challenge)
Cannot attach to process 7196: No such process (3)
Exiting

But when connecting via shell and running, it worked:

$ docker exec -it challenge /bin/bash
root@e2cd6b6e2e2c:/# gdbserver :9090 --attach $(pidof challenge)
Attached; pid = 201
Listening on port 9090

If anybody finds a fix, please let me know!